Endothermic particulate hydroxides have been identified as commercial or potentially commercial flame retardants in polymers, paper and other matrices in the literature. However, different materials have temperature constraints which limit the variety of polymer systems within which they can be used. These temperature limitations include the temperature at which the polymer system is processed and the temperature at which the flame retardant additive begins to decompose and act as a flame retardant.
For example, aluminum trihydroxide (alumina trihydrate) is a well-known endothermic flame retardant additive or filler which has been found to be effective in several important polymer systems. It is commonly used at loadings of up to 75 weight percent of the polymer. Although alumina trihydrate is effective, its use is limited to polymers that are processed at temperatures below 220.degree. C., the point where its water of hydration begins to evolve. At processing temperatures above about 220.degree. C., alumina trihydrate decomposes to alumina and water vapor. The water vapor produces unacceptable surface defects and porosity in the finished product, and can damage processing equipment. Water vapor and/or steam which builds up in the processing equipment can cause both mechanical failure and injury to equipment operators.
It is the heat capacity and water of hydration of aluminum trihydroxide that makes it effective as a flame retardant additive. To maintain burning in plastic systems, three ingredients need to be supplied: heat, fuel and oxygen. Alumina trihydrate is effective as a flame retardant because its endothermic decomposition acts as a cooling heat sink to remove heat from the plastic--heat which would ordinarily go toward decomposing the plastic into the low molecular weight gaseous elements needed to sustain combustion. Water liberated from the decomposition of the hydrate also serves to inhibit the access of oxygen to the plastic systems and dilute the combustible gases. Thus, the addition of alumina trihydrate transforms an exothermic, flame-propagating polymer system into an endothermic, flame-retarding system.
Some of the common polymers for which alumina trihydrate finds use as a flame retardant include polyurethane, polyethylene, varieties of polypropylene having low processing temperatures and some unsaturated polyesters. Alumina trihydrate is compatible with these polymers because the endothermic decomposition temperature of the alumina trihydrate (about 220-300.degree. C.) is above their processing temperatures.
Alumina trihydrate is not presently used as a flame-retardant additive in several other classes of polymers, including for example polyethylene terephthalate (PET), polybutylene-terephthalate (PBT), acrylonitride-butodienestyrene(ABS), nylon, and varieties of polypropylene having high process temperatures. When the trihydrate is added to such polymers, the trihydrate can decompose to alumina and water vapor during processing. The water vapor produces foaming or voids in the polymer matrix. These voids produce surface defects which are unacceptable for most final products.
Many other inorganics possessing "water of hydration" have not been widely used as flame retardants because their water releasing endothermic reactions occur at too low a temperature. One such material is hydrotalcite which is a magnesium aluminum carbonate hydroxide material having the general formula: Mg.sub.x Al.sub.2 (OH).sub.2x+4 (CO.sub.3).yH.sub.2 O where x varies from 3 to 6 and y varies from 2 to 4. Hydrotalcite has the potential of being a good flame retardant because it possesses water of hydration like hydrated alumina (alumina trihydrate). However, it has not found widespread commercial acceptance as a flame retardant because it has a low temperature endotherm peak at approximately 219.degree. C. (see FIG. 2) and liberates 15 wt.% water when heated to 200.degree. C. (see FIG. 3).
Attempts have been made in the past to calcine hydrotalcite at a temperature above the processing temperature of the polymer system in which it will be used. The calcined hydrotalcite could then be used as a flame retardant additive which releases its water of hydration at temperatures above the processing temperature of the polymer and below the decomposition temperature of the polymer. The calcined hydrotalcite must either be used immediately after calcining or stored in a manner that will prevent it from reverting back to its original composition.
U.S. Pat. No. 4,883,533 issued to Kosin et al discloses a phosphate containing hydrotalcite for improving the flame retardant characteristics of plastic systems such as polypropylene.
U.S. Pat. No. 4,351,814 issued to Miyata et al discloses hydrotalcites having a hexagonal needle-like structure and process for production thereof. The hydrotalcite compounds have the formula: Mg(OH).sub.2-n'x2 A.sub.x2.sup.n-.m.sub.2 H.sub.2 O where A.sub.x/n.sup.n- represents anions which include bromide (Br-).
U.S. Pat. No. 4,562,295 issued to Miyata discloses a method of purifying cyclohexanone containing by-product organic acids. The method includes contacting the cyclohexanone with a hydrotalcite having the formula: Mg.sup.2+ M.sub.x.sup.3+ (OH).sub.2x+6-nz (A.sup.n-).sub.z.mH.sub.2 O where A.sup.n- represents anions which include bromide (Br.sup.-).
U.S. Pat. No. 4,642,193 issued to Miyata et al discloses a method for purification of the cooling water used in nuclear reactors. The method includes contacting the cooling water with a hydrotalcite having the formula: Mg.sub.1-x.sup.2+ M.sub.x.sup.3+ (OH).sub.2 A.sub.x/n.sup.n-.mH.sub.2 O where A.sub.x/n.sup.n- represents anions which include bromide (Br.sup.-).
U.S. Pat. No. 4,710,511 issued to Miyata discloses a process for producing a vinyl chloride polymer or copolymer in aqueous suspension using a hydrotalcite compound as suspension stabilizer. The hydrotalcite compounds have the formula: Mg.sub.1-x.sup.2+ M.sub.x.sup.3+ (OH).sub.2 A.sub.x/n.sup.n-.mH.sub.2 O where A.sub.x/n.sup.n- represents anions which include bromide (Br.sup.-).
U.S. Pat. No. 4,710,551 issued to Miyata discloses a process for producing a vinyl chloride polymer or copolymer in aqueous suspension using a hydrotalcite compound as suspension stabilizer. The hydrotalcite compounds have the formula: Mg.sub.1-x.sup.2+ M.sub.x.sup.3+ (OH).sub.2 A.sub.x/n.sup.n-.mH.sub.2 O where A.sub.x/n.sup.n- represents anions which include bromide (Br.sup.-).
U.S. Pat. No. 4,904,457 issued to Misra discloses a method for producing synthetic hydrotalcite by reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions.
There currently exists a need for low-cost additive materials that can be used as flame retardants and smoke suppressants in polymer systems processed at higher temperature. There also exists a need for low-cost materials that have endotherms which begin at temperatures that are higher than alumina trihydrate. The endotherms of these materials must be matched to the processing temperatures and/or exothermic characteristics of each polymer system involved.
In addition, a need exists for a method of converting materials that evolve gas at temperatures which are detrimental to the processing of polymer systems, such as hydrotalcite, into materials that do not evolve gas at temperatures which are detrimental to the processing of polymer systems and possess a flame retarding endotherm below the decomposition temperature of the polymer.
The principal object of the present invention is to provide additive material for use as a flame retardant and smoke suppressant in polymer, paper and other matrices.
Another object of the invention is to provide an additive material for use as a flame retardant in polymer systems which possesses enhanced thermal stability.
Another object of the invention is to provide an additive material for use as a smoke suppressant in polymer systems.
Another object of the present invention is to provide a method of creating a new additive material for use as a flame retardant in polymer systems which is designed to meet the specific needs of that polymer system.
Additional objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.